The present invention relates to switching modulators and demodulators, and more particularly discloses switching modulators and demodulators utilizing switching signals which have been modified so that the output of the switching circuits have reduced harmonic content.
The composite stereo signals generated and transmitted by FM broadcast stations include two components; a baseband component corresponding to the sum of the two source signals (usually referred to as Left (L) and Right (R) signals), and a subcarrier component, modulated on a 38 kHz subcarrier, corresponding to the difference between the two source signals.
Switching modulators have long been used to combine the two audio source signals to produce the composite stereo signal. These switching modulators operate to alternately sample the L and R signals at the subcarrier rate of 38 kHz to produce a time division multiplexed (TDM) signal. Patents embodying this technique include Wilhelm, U.S. Pat. No. 3,040,132, Anderson et al, U.S. Pat. No. 3,789,323, and Bruene, U.S. Pat. No. 3,902,019.
Unfortunately, the signals provided by these switching modulators do not have exactly the same form as that required of the composite stereo signal which is to be transmitted. More specifically, switching modulators not only generate the required baseband and subcarrier components, but also incidentally generate a number of undesirable harmonic components. These harmonic components must be eliminated from the signal prior to transmission, or they would result in unacceptable interference with adjacent radio channels. Although low-pass filters may be used to selectively filter out these higher harmonics, such filters are both expensive and complicated. Consequently, systems have been proposed wherein the unwanted harmonic component closest in frequency to the subcarrier component (i.e., the third harmonic, or 114 kHz component) is eliminated through the addition of another signal having a component which will exactly cancel this harmonic component. Although a filter is still necessary to remove the other, higher harmonic components, the constraints on this filter are reduced since it is no longer necessary to attenuate the harmonic component immediately adjacent the subcarrier. In order to accomplish this harmonic cancellation, however, it has heretofore been necessary to include a second switching modulator in the system.
Demodulators have been implemented utilizing switching circuits in much the same manner, and with much the same problem. Thus, switching demodulators have been used to recover the L and R signals from the FM composite stereo signal, however they also recover any signal incidentally centered on an odd harmonic of a switching frequency. Low-pass filters were thus provided prior to the switching demodulator so as to eliminate any noise at these harmonic frequencies. As in the modulators, such a filter must remove the third harmonic noise component without distorting the fundamental component.